Scanning objective

ABSTRACT

A high resolution with at the same time high testing speed is required of scanning objectives for the linewise or pointwise three-dimensional scanning of object surfaces. In order to be able to image as many image points as possible with high resolution, the scanning objective must exhibit a correspondingly large numerical aperture with, at the same time, a large image field. For this purpose, it is constructed of three lens groups (1; 2; 3). The first and second lens groups (1; 2) effect both a scan angle reduction and a pupil enlargement. The entrance pupil (61) is imaged into the entrance pupil (63) of the third lens group (3). The third lens group (3) has a large numerical aperture of, for example, 0.6. Furthermore, the real intermediate image (5) is imaged by the second lens group (2) with positive refractive power and greater focal length (f) than the first lens group (1), at infinity. With a beam diameter (D) at the entrance of the scanning objective of 7.5 mm, a scan angle (θ) of +/-16° and a scan length S of 3.5 mm, it is possible to achieve, for example, a working spacing of 5 mm.

The invention relates to a scanning objective for the linewise orpointwise three-dimensional scanning of object surfaces with highresolution and high testing speed.

The increasing packing density on an electronic assembly requires acorrespondingly adapted testing. As a rule, this testing takes place bymeans of a high-resolution optical inspection system. In principle, thetriangulation principle can be used in conjunction with a system for therapid scanning of a surface. However, as a rule in the case ofreflecting surfaces of structured objects, the confocal principle ismore suitable. In this case, a point light source which is usuallydefined by an aperture diaphragm, is imaged onto the object surface andthe back-scattered light is imaged onto what is almost a point detector.In this connection, reference is made by way of example to the Europeanpatent application bearing the official file reference 91 120 863.5. Thedepth of field of a confocal optical configuration is a measure of theheight resolution of the system. In this case, the depth of field isinversely proportional to the square of the numerical aperture. Ascanning objective having a high numerical aperture and a very rapidbeam deflection unit, for example a polygonal mirror rotating at a highspeed of rotation, satisfies the requirements with respect to highresolution for an automatic testing system. At the present time,however, a scanning objective having a sufficiently high numericalaperture is not available. A sufficiently high numerical aperture shouldexhibit at least a value of 0.15.

The quantity which is essential in the design of a diffraction-limitedscanning objective, and which reproduces the theoretically attainableimaging performance, is the Lagrange invariant L. This is formed by theproduct of half the beam diameter D at the location of a beam deflectionunit and the scanning angle or, respectively, deflection angle θ. Thisis synonymous with the product of numerical aperture NA and half thescan length S of the scanning objective.

    L=1/2D·θ=1/2S·NA

Since the resolving power is predetermined by the numerical aperture, Lis proportional to the number of scanned points per scan line. A highscan rate is thus achieved by a high speed of deflection in the courseof scanning and the greatest possible Lagrange invariant L of thescanning system. In accordance with the abovementioned dependencies,both beam deflection unit and scanning objective must be adapted to oneanother.

With respect to the beam deflection unit, it results that for rotatingpolygonal mirrors as compared with other beam deflectors, the feasibleangular velocity, with at the same time a large beam diameter, is veryhigh and thus the pixel data rate, is very high. Other beam deflectorsare understood to include, for example, acousto-optic deflectors,resonance scanners or galvanometer mirrors. A limitation of the datarate in an upward direction is provided by the increasing moment ofrotational inertia with increasing beam cross-section or increasingmirror facet diameter of the mirror. Depending upon the material usedand the mechanical construction of the polygonal mirror, the result isaccordingly an optimal dimensioning of the polygonal mirror.

The fundamental design of a scanning objective must take into accountthe following dependencies:

The greater the Lagrange invariant (high resolution with large imagefield), the more difficult it is to minimize geometric imaging defectsand to design a scanning objective in production-oriented fashion.

In the event of an intensification of the requirements on theresolution, which is synonymous with an enlargement of the numericalaperture, the scan length is reduced with an unchanged Lagrangeinvariant.

If, for a system with a predetermined Lagrange invariant, the numericalaperture is increased with constant focal length, then the beam diameterincreases with a reduction of the scan angle. An increase in the beamdiameter at a small scan angle does, however, increase the number andthe size of the facets of the polygonal mirror, whereby again theattainable speed of rotation is reduced.

If, for a system with a predetermined Lagrange invariant, the numericalaperture is increased while maintaining the scan angle, then theobjective focal length is reduced. This means that in the case ofconventional scanning objectives, the front and rear focal planes passvery close to the lens mount, since otherwise the correction of theaberrations is made considerably more difficult. This again limits therequired space for the positioning of polygonal mirrors.

The interrelationships which have been enumerated illustrate that ascanning objective having a large numerical aperture and at the sametime a large image field cannot be constructed using conventional means.

Scanning objectives which are currently available on the market exhibitnumerical apertures of approximately 0.1 and have a scan angle ofapproximately 13° to 25° at focal lengths of at least 20 mm. Theseobjectives can be adapted very well to polygonal mirrors rotating athigh speed. In this case, the Lagrange invariant is approximately 1 mm.Objectives of this type of construction are, however, not suitable forconfocal scanners with high resolution.

Furthermore, confocal laser scanning microscopes exist. In most cases,these are constructed on the basis of conventional optical microscopesor optical microscope objectives, and thus can also possess a highnumerical aperture. With a Lagrange invariant of approximately 0.15 mm,however, the overall imaging performance is relatively low, on accountof the small image field or, respectively, of the small scan length. Inthis case, acousto-optic beam deflectors are frequently used, wherebythe imaging performance is reduced on account of the small deflectionangle and the simultaneously occurring small aperture, which is notcircular. Moreover, the beam deflection is not free from aberration(astigmatism).

German laid-open specification DE 36 10 165 discloses an opticalscanning microscope. This includes two beam deflection means for the twodimensional scanning of an object. The described microscope exhibits ahigh resolving power, which is not, however, combined with a large imagefield with, at the same time, a high scan rate for the scanning of thesurface.

SUMMARY OF THE INVENTION

An object of the invention is the construction of a scanning objective,by means of which at high resolution, corresponding to a high numericalaperture, a large image field is scanned with at the same time a highscanning rate.

According to the invention, a scanning objective is provided which ispositioned downstream of a beam deflection unit for guidance of anillumination beam or an illumination beam and a measurement beam. Thescanning objective is especially useful for a scanning system forsurface inspection of electronic circuit board assemblies. Three lensgroups are provided. The first lens group is provided for coupling tothe beam deflection unit. It has a positive refractive power andgenerates an intermediate image with a numerical aperture which is smallin relation to an entire scanning objective, and has at the same time alarge scan angle. The third lens group is provided on an object side andhas a numerical aperture which is larger relative to the numericalaperture of the first lens group. The second lens group is interposedbetween the first and third lens groups and has a positive refractivepower and a focal length which is greater than a focal length of thefirst lens group. The second lens group images an intermediate imagegenerated by the first lens group at infinity. An exit pupil of thefirst and second lens groups is positioned ahead of the third lensgroup. At a location of an entrance pupil of the third lens group, ascan angle reduction is achieved by the first and second lens groups sothat the third lens group similar to a collimator optical system can becorrected in a simple manner with respect to geometric imaging defects,with the exception of spherical aberation.

In principle, a high resolution of an objective is assured by a highvalue of the numerical aperture. The scan angle or respectivelydeflection angle determines the size of the image field, in conjunctionwith the focal length of the objective.

The invention is based on the finding that by means of a scanningobjective comprising three lens groups, with appropriate design, therequired quality features are achieved. A first lens group, seen in thedirection of the scan beam, having positive refractive power and havinga relatively small numerical aperture, can be considered, in the case ofhigh imaging performance, as an ideal element for coupling to the beamdeflection unit. This optical system corresponds in its construction toa typical scanning objective from the prior art, with a relatively largenumerical aperture. It can be manufactured in diffraction-limitedfashion using fewer than 6 individual lenses. A real intermediate imageis generated by this lens group. Of great advantage is the sufficientzenith distance between the entrance pupil and the surface of the firstlens.

A third lens group, which is disposed on the object side, has a highnumerical aperture with large focal length and with a relatively smalldeflection angle of approximately 2.5°. In order to remain within thediffraction limitation, it must be ensured that geometric imagingdefects are optimally corrected. If required, a reduction of thedeflection angle has to be tolerated. As a result of the similarity ofthe construction to a collimator optical system, essentially only thecorrection of spherical aberration still needs to be undertaken. As aresult of the described design of the third lens group, the imagingperformance of a scanning system or of a scanning objective on theobject side with a high numerical aperture is possible.

A second lens groups having positive refractive power, which images thereal intermediate image of the first lens group at infinity, must beconstructed so that the exit pupil, formed by the first and second lensgroups, coincides with the entrance pupil of the third lens group. Theimage angle of the second lens group must in this case be in accordancewith the deflection angle of the third lens group. This second lensgroup can, for example, be an inverted scanning objective. This meansthat a hitherto known scanning objective is used in the oppositedirection. In this case, it is necessary to ensure the appropriateadaptation of the scan length (image height of the intermediate image)and the numerical aperture with respect to the first lens group.Furthermore, it is necessary to undertake an adaptation with respect tothe scan angle (image angle) and the diameter of the entrance pupil ofthe third lens group. The ratio of the focal lengths of the first lensgroup to second lens group corresponds in this case to the ratio betweenscan angle (image angle) of the first lens group and the third lensgroup. All three lens groups are diffraction-limited.

A particularly advantageous refinement of the invention includes thedesign of the scanning objective as a F-θ objective. The necessarycondition for an F-θ objective is the proportionality between the scanangle or deflection angle and the instantaneous out-deflection or theinstantaneous image height. This has the effect of reducing thetechnical control expenditure in automatic testing systems.

It is advantageous to allow the principal rays to run parallel to theoptical axis between the first and the second lens groups. Thisconstruction, corresponding to a telecentric design, can apply first tothe first and second lens groups and on the other hand to the entirescanning objective. The advantages resulting therefrom reside in thefact that it is possible to connect individual telecentric lens groupsone behind the other in a simple manner. The telecentric design of theentire scanning objective means, in particular, is an advantageousapplication for measurement purposes. Costly electronic (hardware orsoftware) corrections are superfluous.

In the event that the total focal length of the scanning objective issmaller than the spacing between the first lens surface of the firstlens group and the entrance pupil lying in front thereof, of thescanning objective, the coupling between the scanning objective and thebeam deflection unit can be carried out in a simple manner.

If the numerical aperture of the first lens group is at least 0.15 (thenumerical aperture of the entire scanning objective can be, for example,0.6), then this is advantageous insofar as a greater freedom of designwith regard to a high total numerical aperture is attainable for theconstruction of the lens groups which follow.

A further advantageous refinement of the invention provides that thescan angle has a value of approximately 16° and the entrance pupilpossesses a diameter of approximately 7.5 mm. The specification of thesetwo parameters of the scanning objective at the aforementioned numericalvalues gives particular advantages with respect to the design of apolygonal mirror rotating at a high speed of rotation (e.g. number offacets 12, mirror diameter approximately 70 mm, facet lengthapproximately 19 mm).

If the numerical aperture of the third lens group is at least 0.25 incombination with a scan angle of approximately 2.5° and a scan length ofapproximately 8 mm, then it is already possible to set a working spacingbetween objective and object of approximately 30 mm.

In the text which follows, an embodiment is described with reference todiagrammatic figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a basic sketch of a typical scanning objective having anumerical aperture of 0.1.

FIG. 2 shows the beam path of a scanning objective with an enlargementof the numerical aperture as compared with the representation in FIG. 1.

FIG. 3 shows the detailed structure of a scanning objective of highnumerical aperture.

FIG. 4 chows the basic representation of the scanning objectiveaccording to FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The beam path of an objective 11 represented only by a line is outlinedin FIG. 1. The objective 11 is positioned with respect to an opticalaxis 10. The focal length f is indicated on both sides of the objective11. Proceeding from the beam diameter D at the entrance pupil 61,principal ray 7 and marginal rays 8 extend in accordance with the scanangle 8. The object plane 9 is indicated in the right hand part of theillustration. The scan length S corresponds to the image size.

Known scanning objectives represented in FIG. 1 are distributedcommercially by a plurality of manufacturers. As a rule, they exhibit anumerical aperture of <=0.1 and focal lengths of more than 20 mm. Thescan angles lie in the range from +/-13° to 25°. As the Lagrangeinvariant represents a measure of the imaging performance of an opticalcomponent, the numerical aperture determines the size of the image pointand the imaging performance is based on the number of resolved imagepoints, the dimensioning should always be such that the desired apertureis present. If it is assumed that the Lagrange invariant remainsconstant as a measure of the imaging performance and the focal length ofthe entire system does not change, then upon increasing the numericalaperture the result is a beam path corresponding to FIG. 2. In this casealso, the entrance pupil 61 and the object plane 9 lie in each instanceat the focal point. The scan length S is outlined correspondinglysymmetrically in relation to the optical axis 10 and the beam diameter Dat the entrance pupil 61, which diameter is substantially greater thanin FIG. 1, is shown. It is clearly evident that the scan angle θ hassubstantially decreased in relation to the scan angle θ of FIG. 1. Thesame applies to the scan length S. Principal ray 7 and marginal rays 8have likewise been shown. A construction corresponding to FIG. 2 is,however, not practicable for the achievement of the set object, since inthis case the accommodation of a beam deflection unit is, as a rule,possible only under poor conditions. The construction itself correspondsapproximately to a collimator optical system, which as a rule imagesonly one point. In this case, a high numerical aperture becomesattainable in a simple manner, since the image angle may be small.

If, based on the representation in FIG. 1, the Lagrange invariant andthe scan angle θ are kept constant and the numerical aperture isincreased, then the focal length f becomes smaller. This likewisepresents difficulties in relation to the accommodation of a beamdeflection unit ahead of the scanning objective.

The use of a pure microscope objective, for example 50×/0.6, is likewisepossible only with difficulty for the achievement of the set object,since in this case an excessively small deflection angle is present.Beyond above this, again the entrance pupil is usually situated veryclose to the lens system of the microscope objective.

FIG. 3 shows a scanning objective with an upstream beam deflection unit4. The scanning objective, which is built up from the described lensgroups 1 to 3, possesses a numerical aperture of 0.6. The first lensgroup 1 and the second lens group 2 comprise in each instance 5individual lenses. The first lens group possesses a numerical apertureof 0.15 and a focal length of 25 mm. The second lens group collimatesthe beams emanating from the real intermediate image 5 at a focal lengthof 160 mm. In this case, a beam expansion and an angle reduction takeplace. The third lens group 3, which comprises a doublet, three directlyadjoining, positively refracting individual lenses and a lens triplet,possesses a focal length of 40 mm and a numerical aperture of 0.6. Theoverall length of the objective is approximately 650 mm.

A scanning objective according to FIG. 3 possesses a high numericalaperture (0.6 and a large scan length S). The first lens group i havingan even smaller numerical aperture of 0.15 exhibits a large scan angleof approximately +/-16°. The first lens group 1 represents the couplingelement to the beam deflection unit 4. The high numerical aperture of0.6 of the third lens group 3 having a positive refractive power isattained by a small image angle of 2.5°. This means, for example, forthe third lens group 3, a pupil diameter of 48 mm, a focal length f of40 mm and an image field of 3.5 mm. In this case, with respect to itsconstruction, this lens group is similar to a larger-scale microscopeobjective having a 50-fold magnification.

In principle, the first two lens groups 1, 2 thus effect a reduction ofscan angle and a pupil enlargement (seen from left to right in thefigure). In this case, the entrance pupil 61 of the first lens group 1is imaged into the entrance pupil 63 of the third lens group 3, or,respectively, the exit pupil 62 of the first and second lens group 1, 2coincides with the entrance pupil 63 of the third lens group 3. Thisgives the advantage that the spacing between entrance pupil 63 and theglass surface of the first lens of the third lens group 3 is released asan additional degree of freedom in the construction and for theoptimization of the third lens group 3. The consideration of pupils ofindividual lens groups is applied only for the construction of ascanning objective. Such a scanning objective has, overall, only oneentrance pupil and one exit pupil. An additional telecentricconstruction of the scanning objective simplifies the measured dataevaluation of an automatic 3D inspection system.

Corresponding to FIG. 3 or 4, the intermediate image 5 is generated bythe lens group 1. The path of the principal ray 7 and of the marginalrays 8 with respect to the lens groups 1-3 and with respect to theoptical axis 10 is indicated. The second lens group 2 images theintermediate image 5 at infinity. In this case, a pupil imaging takesplace by the first and second lens groups 1, 2, in which case theentrance pupil 61 of the first lens group 1 is imaged into the entrancepupil 63 of the third lens group 3. This can also be designated as scanangle transformation. The entrance pupil 63 of the third lens group 3lies between the second and third lens groups 2, 3. Parallel rays areconducted through the pupil at maximum diameter. The first lens group 1,considered as such, can be considered as a scanning objective of lowresolution. The same applies to the second lens group 2; in this case,however, this exhibits a greater focal length f.

Using a described scanning objective, for example, resolution can takeplace within the range of 0.5 μm.

FIG. 4 shows a basic representation of a scanning objective having ahigh numerical aperture.

Possible Technical Data EXAMPLE 1

Numerical aperture (overall): 0.6

Focal length f: 6.25 mm

Beam diameter D, at the entrance: 7.5 mm

Working spacing: 5 mm

Scan angle θ: +/-16°

Spacing a: 15 mm

Scan length S: 3.5 mm

EXAMPLE 2

Beam diameter and scan angle are constant

Numerical aperture: 0.28

Scan length S: 7.5 mm

Working spacing: 27 mm

Focal length f: approx. 14 mm

The basic construction of a scanning objective according to theinvention includes, corresponding to the diagrammatic representation inFIG. 4, a first, second and third lens group 1, 2, 3. The surface of theobject to be tested or the object plane 9 is outlined at the right handmargin of the illustration. The entrance pupil 61 of the entire scanningobjective is represented at the left hand margin of the illustration.The entire arrangement exhibits an optical axis 10. The characteristiccourse of the principal ray 7 and of the marginal rays 8 iscorrespondingly outlined.

The entrance pupil 61, represented in FIG. 4 as a diaphragm, can bereplaced by a rotating polygonal mirror. The exit pupil 62 of the firstand second lens groups 1, 2 coincides with the entrance pupil 63 of thethird lens group 3; in this case, the second lens group 2 images theintermediate image 5, generated by the first lens group 1, at infinity.

Although various minor changes and modifications might be proposed bythose skilled in the art, it will be understood that I wish to includewithin the scope of the patent warranted hereon all such changes andmodifications as reasonably come within my contribution to the art.

I claim:
 1. A scanning objective for being positioned downstream of abeam deflection unit for guidance of an illumination beam for a scanningsystem, comprising:first, second, and third lens groups: the first lensgroup being positioned for coupling to said beam deflection unit andhaving a positive refractive power for generating an intermediate imagewith a numerical aperture which is smaller in relation to the entirescanning objective and at the same time having a large scan angle; thethird lens group being positioned at an object side and having anumerical aperture which is larger than said numerical aperture of saidfirst lens group; said second lens group being interposed between thefirst and third lens groups and having a positive refractive power and afocal length which is greater than a focal length of said first lensgroup and which images said intermediate image generated by the firstlens group at infinity; an exit pupil formed by the first and secondlens groups being positioned ahead of said third lens group; said firstand second lens groups providing a scan angle reduction at a location ofan entrance pupil of said third lens group compared to said large scanangle in front of said first lens group so that the third lens group,which is similar to a collimator optical system, can be corrected in asimple manner with respect to geometric imaging defects, except forspherical aberration; and the scanning objective being an F-θ objective.2. A scanning objective for being positioned downstream of a beamdeflection unit for guidance of an illumination beam for a scanningsystem, comprising:first, second, and third lens groups; the first lensgroup being positioned for coupling to said beam deflection unit andhaving a positive refractive power for generating an intermediate imagewith a numerical aperture which is smaller in relation to the entirescanning objective and at the same time having a large scan angle; thethird lens group being positioned at an object side and having anumerical aperture which is larger than said numerical aperture of saidfirst lens group; said second lens group being interposed between thefirst and third lens groups and having a positive refractive power and afocal length which is greater than a focal length of said first lensgroup and which images said intermediate image generated by the firstlens group at infinity; an exit pupil formed by the first and secondlens groups being positioned ahead of said third lens group; said firstand second lens groups providing a scan angle reduction at a location ofan entrance pupil of said third lens group compared to said large scanangle in front of said first lens group so that the third lens group,which is similar to a collimator optical system, can be corrected in asimple manner with respect to geometric imaging defects, except forspherical aberration; and said first lens group having a first lenssurface and wherein a spacing between a zenith of said first lenssurface and an entrance pupil lying in front of said scanning objectiveis greater than a total focal length of the scanning objective.
 3. Ascanning objective for being positioned downstream of a beam deflectionunit for guidance of an illumination beam for a scanning system,comprising:first, second, and third lens groups; the first lens groupbeing positioned for coupling to said beam deflection unit and having apositive refractive power for generating an intermediate image with anumerical aperture which is smaller in relation to the entire scanningobjective and at the same time having a large scan angle; the third lensgroup being positioned at an object side and having a numerical aperturewhich is larger than said numerical aperture of said first lens group;said second lens group being interposed between the first and third lensgroups and having a positive refractive power and a focal length whichis greater than a focal length of said first lens group and which imagessaid intermediate image generated by the first lens group at infinity;an exit pupil formed by the first and second lens groups beingpositioned ahead of said third lens group; said first and second lensgroups providing a scan angle reduction at a location of an entrancepupil of said third lens group Compared to said large scan angle infront of said first lens group so that the third lens group, which issimilar to a collimator optical system, can be corrected in a simplemanner with respect to geometric imaging defects, except for sphericalaberration; and said numerical aperture of said first lens group beingat least 0.15.
 4. A scanning objective for being positioned downstreamof a beam deflection unit for guidance of an illumination beam for ascanning system, comprising:first, second, and third lens groups; thefirst lens group being positioned for coupling to said beam deflectionunit and having a positive refractive power for generating anintermediate image with a numerical aperture which is smaller inrelation to the entire scanning objective and at the same time having alarge scan angle; the third lens group being positioned at an objectside and having a numerical aperture which is larger than said numericalaperture of said first lens group: said second lens group beinginterposed between the first and third lens groups and having a positiverefractive power and a focal length which is greater than a focal lengthof said first lens group and, which images said intermediate imagegenerated by the first lens group at infinity; an exit pupil formed bythe first and second lens groups being positioned ahead of said thirdlens group; said first and second lens groups providing a scan anglereduction at a location of an entrance pupil of said third lens groupcompared to said large scan angle in front of said first lens group sothat the third lens group, which is similar to a collimator opticalsystem, can be corrected in a simple manner with respect to geometricimaging defects, except for spherical aberration; and said scan anglehaving a value of approximately 16° and a beam diameter at an entrancepupil in from of the scanning objective being approximately 7.5 mm.